In communications systems, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications system is deployed.
For example, transmission schemes and reception schemes based on the use of narrow beams might be needed at high frequencies to compensate for propagation losses. For a given communication link, a beam can be applied at both the network side (such as at the transmission and reception point (TRP) of a network node) and the user side (such as at wireless devices served by the network node). A beam pair link (BPL) is defined by the beam used by the TRP (denoted TRP beam) for communicating with the wireless device and the beam used by the wireless device (denoted WD beam) for communicating with the TRP. Each of the TRP beam and the WD beam could be used for any of transmission and reception. Likewise, there could be separate BPLs for downlink communications (where the TRP beam is a transmission (TX) beam and where the WD beam is a reception (RX) beam) and uplink communications (where the TRP beam is an RX beam and where the WD beam is a TX beam).
In general terms, a beam management procedure is used to discover and maintain BPLs. A BPL is expected to be discovered and monitored by the network using measurements on downlink reference signals used for beam management, such as channel state information reference signals (CSI-RS). The CSI-RS for beam management can be transmitted periodically, semi-persistently or aperiodic (such as being event triggered) and they can be either shared between multiple wireless devices or be device-specific. In order to find a suitable TRP beam the TRP transmits CSI-RS in different TRP TX beams on which the wireless devices performs reference signal received power (RSRP (measurements and reports back the N best TRP TX beams (where the value of N can be configured by the network). Furthermore, the CSI-RS transmission on a given TRP TX beam can be repeated to allow the wireless device to evaluate suitable WD beams, thus enabling so-called WD RX beam training.
It is expected that the wireless device might use two or more antenna arrays, preferably pointing in different directions, in order to improve the coverage and increase the order of spatial multiplexing.
FIG. 1 is a signaling diagram of a beam training procedure of a wireless device 200 served by a network node 300. The network node 300 (e.g. by means of a TRP) in step S301 transmits a burst of CSI-RS in the same TRP TX beam to let the wireless device 200 perform WD RX beam training. One CSI-RS is needed for every combination of WD RX beams for the multiple antenna arrays at the wireless device 200. For example, if the wireless device 200 has two antenna arrays, and where each antenna array is configured to generate two RX beams, the network node 300 needs to transmit four CSI-RSs, because there are four different combinations of WD RX beams. In step S302 the wireless device 200 measures the RSRP on the received CSI-RS for each WD RX beam combination by sweeping through one WD RX beam at a time. In step S303 the wireless device 200 selects the WD RX beam combination that gave the highest total RSRP.
In view of the above, when the wireless device 200 has multiple antenna arrays and needs to perform WD RX beam training in order to update which beam to use at each antenna array many different beam combinations have to be evaluated. This requires large overhead signaling in downlink since one occurrence of the reference signal needs to be transmitted for every single WD RX beam that should be evaluated.
Hence, there is still a need for an improved beam training procedure for wireless devices.